This invention relates to fuel cell systems and, in particular, to fuel cell systems which employ anode gas recycling.
In conventional fuel cell systems and, in particular, molten carbonate fuel cell systems, reactant fuel and oxidizing gas are delivered to each of the fuel cells of the system. The fuel is passed through the anode compartment of each fuel cell, while oxidizing gas is passed through the cathode compartment.
As is known, not all of the fuel delivered to the anode compartment of a fuel cell is converted into electrical power. Typically, in a molten carbonate fuel cell, approximately 10 to 50% of the fuel exits the cell as anode exhaust gas. As a result, in order to increase the fuel cell efficiency, it is a conventional practice to recycle a portion or all of the anode exhaust gas back to the input of the anode compartment. Various procedures for recycling the anode exhaust gas have been developed.
In one type of arrangement, the fuel exhaust gas, which contains fuel (usually unreacted hydrogen and carbon monoxide), water vapor and carbon dioxide is processed to separate these constituents. The separated unreacted fuel is then recycled to the anode compartment of the fuel cell, as is a portion of the water which results as a byproduct from fuel consumption. The separated carbon dioxide, on the other hand, may be likewise recycled, in this case to the cathode compartment of the fuel cell. Carbon dioxide recycle is required for molten carbonate type fuel cells. With other fuel cell types, carbon dioxide may be transferred to the cathode exhaust gas, rather than the cathode air supply, and vented. These recycling operations improve efficiency, as above-stated, and enhance fuel cell operation.
A fuel cell system using the above-described recycling is disclosed in commonly assigned U.S. Pat. Nos. 4,532,192. In the system of the '192 patent, a hydrogen transfer device, such as an electrochemical cell, is used to separate the unreacted hydrogen from the anode exhaust gas. A condenser then removes the water. A portion of the removed water is passed through a heat exchanger whose output is recycled to the fuel cell anode compartment along with the separated unreacted hydrogen. The exhaust gas stream remaining after removing the water is then passed to a burner for burning any hydrogen with the oxidant supply gas to produce a resultant oxidant gas stream rich in carbon dioxide. This stream is then passed into the cathode compartment of the fuel cell.
U.S. Pat. Nos. 5,068,159, and 4,039,579 teach another system of this type. In this system, a cooler and condenser are first used to separate water from the anode exhaust stream. The resultant water is then passed through a boiler and a heater and fed to the inlet of the anode compartment. The anode exhaust stream, absent the water, is then processed to remove the carbon dioxide. The resultant stream is then recycled to the anode compartment of the fuel cell, while the removed carbon dioxide is recycled to the cathode compartment of the fuel cell.
The carbon dioxide separator used in the '159 and the '579 patents comprises an absorber, where the carbon dioxide is absorbed by an aqueous amine solution in an absorption column. The resultant solution is then fed to a regeneration column in which the carbon dioxide gas is stripped with air supplied from an air feed duct. The air, now rich in carbon dioxide, is then fed to the cathode compartment of the fuel cell.
As can be appreciated, the systems of the above patents require the use of complex and costly equipment for realizing their recycling and separation operations. In the '192 patent, a fuel cell type hydrogen transfer device is used, as well as heat exchangers and a burner to provide the desired recycling operations.
In the '159 and '579 patents, on the other hand, condensers, a boiler, a heater and a liquid gas separator are used to recycle hydrogen, water and carbon dioxide. The gas separator, moreover, is subject to foaming or carryover of liquid resulting from the to pumping of the liquid absorbent within the system. Moreover, such a system would require compression of the gas and/or heating of the absorbing liquid to be practical. Compressing and/or heating consume significant energy, making this option relatively unattractive. The separator also requires the use of two columns and air in order to absorb and then regenerate the carbon dioxide.
In addition to the above types of systems, other types of so-called “pressure swing adsorption systems” have been used to process a composite hydrogen gas from a natural gas reformer to separate the hydrogen from other gases. In a standard pressure swing adsorption (“PSA”) system the composite gas is typically fed to the PSA system at pressures of more than 100 psia. The gases other than hydrogen (e.g. carbon dioxide and water) are adsorbed by the adsorbent bed media at high pressures and a pure hydrogen stream exits the PSA system at a pressure close to the inlet pressure. After the adsorbent bed media in the PSA system reaches its maximum adsorbent capacity, it must be purged to remove the adsorbed gases. This occurs by de-sorption which is accomplished by lowering the pressure to near atmospheric pressure of about 20 psia.
Another conventional pressure swing adsorption apparatus is a vacuum pressure swing adsorption (“VPSA”) system, which operates at atmospheric pressures. In VPSA systems, de-sorption is carried out by lowering the pressure to create pressure vacuum conditions.
The conventional PSA systems require a significant amount of power to operate. A standard PSA system typically consumes 15 to 35% of the recycle fuel value as compression power in order to compress the gas to 100 to 300 psia. Because of the higher value of power energy relative to fuel, this is a substantial penalty. A conventional VPSA system requires a similar amount of power in order to generate vacuum conditions to de-sorb the adsorbent beds.
It is therefore an object of the present invention to provide a fuel cell system having an improved anode exhaust gas recycle assembly.
It is a further object of the present invention to provide a fuel cell system having an anode exhaust gas recycle assembly which is less complex and costly and is more energy efficient.